This invention relates to pipeline pigs, and more particularly to such pigs for inspecting the condition of metallic pipelines to determine corrosion or the like.
Current pipeline inspection vehicles are usually one of two main types. The first type incorporates a fixed central body and a xe2x80x98sweeps brushxe2x80x99 arrangement with long bristles which resiliently engage the inner wall of the pipeline.
The second type comprises a central body carrying a plurality of circumferentially spaced, individual sprung elements each provided with relatively short bristles which are urged into engagement with the inner wall of the pipeline, the elements creating a plurality of magnetic return paths the change in flux in which is used to determine the condition of the pipeline.
Both the above types of inspection vehicles are capable of monitoring the condition of pipelines of relatively constant diameter, while the second type can also be used in pipelines with different diameters within the range of sprung movement of the return path elements, providing the wall of the pipeline is continuous.
However, problems arise with this second type when, for example, passing T-connections to the pipeline under inspection, or if there are dents or non-circular portions to the pipeline. In such circumstances, and as the pig passes the T-connection, some of the sprung return path elements are released into the T-connection, and can suffer considerable damage on impact with the wall of the connection.
It would be desirable to be able to provide a pipeline pig capable of inspecting pipelines within a range of diameters and which incorporate T-connections and the like, and dents or other non-circular features without the risk of damage to the pig.
According to the present invention there is provided a pipeline pig including a central body member, a plurality of circumferentially spaced return path elements surrounding said body member and each extending axially relative to the body member, each return path element including a pair of axially spaced magnets from each of which projects a metallic bristle set, and means for connecting the return path elements to the body member whereby the free ends of the bristles, in use, resiliently engage the inner wall of the pipeline, characterised in that said connecting means comprise a first carrier member mounted on, to be axially slidable relative to, the front extent of the body member and resiliently urged towards the rear of the body member, a second carrier member mounted on, to be axially slidable relative to, the rear extent of the body member and resiliently urged towards the front of the body member, and, for each return path element, a front link pivotally interconnecting the front of the return path element and the first carrier member and a rear link pivotally interconnecting the rear of the return path element and the second carrier member.
It will be appreciated that, with such an arrangement, the combined effect of the resilient urging of the first and second carrier members towards each other, in combination with the pivotal linkages between the carrier members and the return path elements, is to urge the bristle sets into engagement with the inner wall of the pipeline.
Furthermore, the nature of the link connections at the front and rear of the pig is such as to ensure that the front ends of the return path elements and the rear ends of the return path elements are constrained to adopt circular configurations in the manner of umbrella-type mechanisms. The diameters of the front and rear circular configurations need not be the same, whereby the pig of the invention is particularly suited to use in a tapering pipeline.
It will also be appreciated that, as the return path elements are constrained to move together with one another, any element passing over the entrance to a T-connection will be prevented from entering that connection, and damage thereto will be prevented.
In the event that one or more return path elements engage with a dent or other non-circular feature on the pipeline inner wall, that element will be moved radially inwardly to accommodate the dent. As a consequence of the linkages, all the other elements will be correspondingly moved radially inwardly whereby the circular configurations of the front and rear ends of the elements are maintained. The lengths of the bristles of the bristle sets are chosen such as to ensure that contact of the free ends of the bristles with the inner wall of the pipeline is maintained for the range of pipeline diameters and dent dimensions anticipated.
In a preferred embodiment of the invention, at least the front and rear extents of the central body member are hollow and of circular cross-section, the front and rear carrier members each being of generally cylindrical shape to be a sliding fit in the front and rear extents of the body member respectively.
Conveniently the front and rear carrier members are resiliently urged towards one another by one or a plurality of coil springs reacting between the front of the body member and the front of the first carrier member and between the rear of the body member and the rear of the second carrier member respectively.
One end of each link is pivotally connected to the associated carrier member and the other end of each link is pivotally connected to the associated return path element, the front and rear extents of the body member being provided with a plurality of axially extending, circumferentially spaced slots one for each link to accommodate the axial sliding movement of the one ends of the links.